update "Trigger registry scans with webhooks" page

[alexshkatov]

Seem like the Trigger registry scans with webhooks page (https://github.com/twistlock/docs/blob/master/admin_guide/vulnerability_management/registry_scanning/webhooks.adoc) is outdated.

Allow image scans initiated by registry webhooks option was deprecated in 2018.

[sgordon46]

@alexshkatov deprecated in 2018 for a specific registry?

[alexshkatov]

@sgordon46 No, for any registry. Starting from the commit https://github.com/twistlock/twistlock/commit/091d4e6e3606437573f468294567c3874156eabe (Jul 11, 2018), registry web hooks are always enabled (no need to turn it ON)

[sgordon46]

@alexshkatov ok, this makes more sense. thanks for the clarification.